Fixes #673, IA-S3 check_limit error message bug.

[jjjake]

Fixed bug where failed requests to IA-S3 check_limit API would be treated as a 503 slowdown error.

[jjjake]

Here's an example of what the output would look like in ia upload for a read-timeout from the IA-S3 check_limit API:

» ia upload jj-test-2024-12-10 internetarchive-5.0.5.tar.gz -H 'X-Archive-Simulate-Error:SlowDown' --retries 3 --sleep 1
jj-test-2024-12-10:
 uploading internetarchive-5.0.5.tar.gz: 100%|████████████████████████████████████████████████| 1/1 [00:00<00:00, 25.01MiB/s]
 warning: s3 is overloaded, sleeping for 1 seconds and retrying. 3 retries left.
 warning: error checking if s3 is overloaded via s3.us.archive.org?check_limit=1, exception raised: "Fake ReadTimeout error: The read operation timed out.". sleeping for 1 seconds and retrying. 2 retries left.
 warning: error checking if s3 is overloaded via s3.us.archive.org?check_limit=1, exception raised: "Fake ReadTimeout error: The read operation timed out.". sleeping for 1 seconds and retrying. 1 retries left.
 uploading internetarchive-5.0.5.tar.gz: 100%|████████████████████████████████████████████████| 1/1 [00:00<00:00, 41.87MiB/s]
 error uploading internetarchive-5.0.5.tar.gz: Please reduce your request rate. - simulated error caused by x-(amz|archive)-simulate-error, try x-archive-simulate-error:help